The rapid extraction events of a control absorber are among the main reactivity incidents in a nuclear reactor. These events may cause an injection of reactivity into the core of the reactor too fast to be detected by the neutron measuring chains and to be processed by the central command control system before a critical accident occurs.
Outside maintenance situations, the main cause of such an event is a failure of the control of the drive mechanism or the electric motor.
It is important to note that any reactor, due to its design, has an anti-reactivity margin β such that the injection of reactivity Δρ0β, in a very short length of time, or even in stages, does not cause any critical neutronic consequence.
Conversely, an excessive injection of reactivity, when it exceeds β, causes very rapid and uncontrollable seizing of the neutronic power (passage to prompt criticality causing doubling of the neutronic power in just a few milliseconds).
Due to the significance of the potential consequences, it is necessary to establish very high safety measures, if one wishes to rule out this event at the design stage.
Furthermore, the speed with which the considered incident may develop makes an a posteriori control by observation of the neutron flux difficult. The time necessary to implement corrective measures upon detecting a sudden increase in the neutron flux ranges from several seconds to several tens of seconds, which is much too late.
This problem is particularly acute in the case of a nuclear reactor designed to:                preclude the ejection of control absorbers under the effect of the hydraulic thrust of the primary fluid, for example if the cover crossmember breaks if the drive mechanism is housed outside the vessel;        preclude the mechanical rising of the control absorber, upon internal or external attack without a force being applied on a mechanism.        
Integrated reactors of the SMR (Small and Modular Reactor) type are designed in this way, in order to improve safety. Typically, they comprise submerged drive mechanisms, with no cover crossmember. As a result, the reactivity injections associated with failures of the control of the drive mechanism are no longer covered by the envelope event for ejection of the control clusters.
Several technical solutions exist to control the rise speed of the control absorbers.
A first possibility is to use pawl mechanisms. These mechanisms have been used for some time, in many nuclear reactors. They mechanically limit the movement speed of the absorbers. A submerged version of a pawl mechanism is disclosed by US 2012/148007. Such submerged mechanisms have significant drawbacks: they are mechanically complex, and have a substantial radial bulk.
It is also possible to use DC rotary motors. The control of the nominal speed and the movement monitoring of the absorbers is provided by similar devices limiting the voltage applied to the motor. In such a mechanism, the rise speed of the absorber depends on the torque. As a result, the operating point of the mechanism can evolve over time with the voltage, the efficiency of the reduction gears, friction, etc. It is thus necessary to account for a significant margin between the nominal speed and the overspeed in the safety studies, typically a factor 3. This is detrimental for the control of the reactor.
It is also possible to use AC rotary motors. In this case, the control of the overspeed is provided by guaranteeing a perfectly bounded frequency of the network, typically 50 or 60 hertz. This is problematic in some countries or for some sites, in which the frequency of the network is not perfectly controlled.
For both types of rotary motor mechanism, the principle to avoid the event considered above consists of controlling the rise speed of the absorbers upon failure, and checking that the associated reactivity injection Δρ0 during the reaction time frame of the control chains of the reactivity of the central command control of the nuclear reactor remains below the prompt criticality threshold β, with a significant margin corresponding to the design options.
This approach raises two problems.
The actual rise speed of the absorbers depends on the ratio between the motor torque and the resisting torque, and is therefore a complex function of multiple parameters: mass, efficiency of the reducing gears, friction, supply voltage, etc.
These parameters evolve over time. It is thus necessary to adopt safety margins that penalize the ability to reconcile a rapid descent speed, and a limited maximum ascent speed.
Furthermore, when the neutron flux is weak, the response time of each of the chains of the central command control is high. This response time is typically from several seconds to several tens of seconds.
Lastly, known from U.S. Pat. No. 4,777,010 is a drive mechanism for at least one control absorber in which the absorber is moved by an electric motor of the stepping type.